1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or a printer or the like that uses an electrophotography scheme or an electrostatic storage scheme or the like. Specifically, the present invention relates to improvements of such an apparatus in the density, tone reproduction and color tint of its toner images.
2. Description of the Related Art
FIG. 13A schematically shows an example of a sensor utilizing a photodiode for detecting light reflected from toner. FIG. 13B shows an example of a circuit that converts an output current of the photodiode into a voltage. In FIGS. 13A and 13B, reference numeral 201 denotes a photodiode, and reference numeral 102 denotes an LED serving as a light source. Reference numeral 104 denotes a toner image (which will be referred to as a xe2x80x9ctoner patchxe2x80x9d hereinafter) to be detected that has been formed on a transferring material. Reflected light 206 from the toner patch is incident on the photodiode 201, so that a photocurrent is generated. The photocurrent is converted into a voltage V203 by a resistance 202. The voltage V203 reflects the quantity of reflected light in real time.
FIG. 14 is a block diagram showing a structure of a conventional accumulation type line sensor. In FIG. 14, reference numerals 204, 205 and 206 denote a sensor array, a readout circuit and a reset circuit respectively. Pixels 207 to 209 and 220 are dark pixels surfaces of which are shielded from light. Pixels 210 to 219 are sensitive to light. Pixels 207 and 220 also serve as dummy pixel that are disposed at the ends of the sensor array to absorb variations in characteristics of the sensor due to their locations at the ends. For the sake of simplicity, an example that includes ten light sensitive pixels is shown in FIG. 14, but it should be noted that the number of effective pixels is determined in accordance with various requirements. In this example, the dark pixels comprise three bits located at the front end and one bit located at the rear end, but it should also be noted that the number of bits would be increased or decreased in accordance with the extent of light leakage between pixels or with requirements posed on the system to be used.
FIG. 15 is a timing chart showing an operation of the accumulation type line sensor shown in FIG. 14. The accumulation is started upon releasing a reset status after the sensors are reset with a reset pulse 221. During the accumulation, accumulation capacities (not shown) of the sensors are charged by photocurrents corresponding to the incident light quantities. However, in those bits which are shielded from the light, the accumulation capacities are charged by dark currents generated in the sensors. After a predetermined time ta of the accumulation, the outputs of the sensors are transferred with a transfer pulse 222 to the readout circuit 205 at one time. Then they are output, pixel by pixel, in a serial manner as an output signal 224, based on shift pulses 223 generated by a shift register in the readout circuit. In this process, the output corresponding to the dark pixel 208 is taken as a representative dark output, so that that output is subtracted from each of the outputs of the subsequent effective pixels to obtain a corrected signal that compensates for an error due to the dark current of the sensor.
A description will be made of the outline of an overall structure of a color laser printer as a multi-color image forming apparatus. The color laser printer forms an electrostatic latent image in an image forming unit in accordance with an image light generated based on an image signal, and then develops the electrostatic latent image so as to form a color visible image, and further transfers the color visible image onto a transfer material as a recording medium, and fixes the color visible image. The image forming unit includes, for stations arranged in parallel corresponding to respective developing colors, photosensitive drums 5Y, 5M, 5C and 5K, injection charging means 7Y, 7M, 7C and 7K as primary charging means, developing means 8Y, 8M, 8C and 8K, and toner cartridges 11Y, 11M, 11C and 11K. The image forming unit also includes an intermediate transfer member 12, a sheet feeding unit, a transferring unit and a fixing unit 13.
The photosensitive drums 5Y, 5M, 5C and 5K are comprised of aluminum cylinders coated with an organic photoconductive layer on their peripheral surfaces. These photosensitive drums are rotated by a driving force transmitted from a driving motor (not shown), which causes the photosensitive drums 5Y, 5M, 5C and 5K to rotate counterclockwise in accordance with image forming operations. Exposing lights for the photosensitive drums 5Y, 5M, 5C and 5K are transferred from scanner units 10Y, 10M, 10C and 10K so as to selectively expose the photosensitive drums 5Y, 5M, 5C and 5K, so that electrostatic latent images are sequentially formed.
As primary charging means for charging the yellow (Y), magenta (M), cyan (C) and black (K) photosensitive drums, four injection charging means 7Y, 7M, 7C and 7K are provided for the respective stations. The injection charging means 7Y, 7M, 7C and 7K are equipped with sleeves 7YS, 7MS, 7CS and 7KS respectively.
As developing means for making the electrostatic latent image visible, four developing devices 8Y, 8M, 8C and 8K that perform developments in yellow (Y), magenta (M), cyan (C) and black (K) are provided for the respective stations. The developing devices 8Y, 8M, 8C and 8K are equipped with sleeves 8YS, 8MS, 8CS and 8KS respectively. Each of the developing devices is detachably mounted on the body of the apparatus.
The intermediate transfer member 12 is composed of an endless belt looping around a driving roller 18a and driven rollers 18b and 18c, which is in contact with photosensitive drums 5Y, 5M, 5C and 5K. At the time of color image formation, the intermediate transfer member 12 is rotated clockwise so as to be sequentially transferred with images by means of primary transferring rollers 6Y, 6M, 6C and 6K of respective colors.
A sheet feeding cassette 2 or a sheet feeding tray 3, which serves as sheet feeding means (or a sheet feeding opening), accommodates transfer materials 1. The transfer materials 1 are transported through a transporting path 25 defined by feed rollers 4, transporting rollers 24 etc. so as to be delivered to registration rollers 23. The arrival of the transfer material 1 at the registration roller 23 is detected by a sensor 19 positioned before the registration roller 23.
During the formation of the image, the transportation of the transfer material 1 is suspended for a predetermined time in synchronization with the time, which is determined based on the detection by the sensor 19 before the registration roller, at which a color visible image on the intermediate transfer material 12 would arrive at a transferring area. The transfer material 1 is fed to the transferring area at which a secondary transferring roller 9 is in contact with the intermediate transfer member 12 to receive and transport the transfer material 1 therebetween, whereby color visible images on the intermediate transfer member 12 are transferred onto the transfer material 1 at one time in a superposed manner.
While the color visible images are transferred in a superposed manner, the secondary transferring roller 9 assumes a position shown by a solid line in FIG. 16, at which the secondary transferring roller 9 keeps the transfer material 1 in contact with the intermediate transfer member 12. However, after completion of the printing process, the secondary transferring roller 9 is shifted apart from the intermediate transfer member 12 to a position shown by a broken line in FIG. 16.
The fixing unit 13 functions to fix the transferred color visible image while transporting the transfer material 1. The fixing unit 13 has a fixing roller for heating the transfer material 1 and a pressurizing roller 15 for pressing the transfer material against the fixing roller 14. The fixing roller 14 and the pressurizing roller 15 have hollow structures and accommodate a heater 16 and 17 respectively. Thus, the transfer material 1 that bears the color visible image is applied with heat and pressure so that the toner is fixed on the surface of the transfer material 1, while the transfer material 1 is transported by the fixing roller 14 and the pressurizing roller 15.
After the fixing of the visible image, the transfer material 1 is discharged to a discharging unit (not shown) by a discharging roller (not shown), so that the image forming operation is completed. The discharge of the transfer material 1 from the fixing unit 13 is detected by a fixing discharge sensor 20.
Cleaning means 21 collects waste toner remaining on the intermediate transfer member 12 after transferring the four color visible images formed on the intermediate transfer member 12 onto the transfer material 1.
Color misregistration detecting means 22 forms a color misregistration detecting pattern on the transfer material 1 and detects misregistration amounts between the colors with respect to a main scanning direction and a sub-scanning direction to provide feedback for fine adjustment of the image data so as to reduce color misregistration.
Changes in conditions of some portions of the image forming apparatus due to changes in environmental conditions or due to long term use of the apparatus bring about changes in the density or chromaticity of the obtained images. Especially in the case of color image forming apparatus using an electrophotography scheme, a slight change in the density can deteriorate the color balance, so it is necessary to always keep a constant density, a constant tone and a constant color tint.
For that purpose, the image forming apparatus is provided with tone correcting means in the form of a look-up table or processing conditions such as a plurality of exposure amounts or developing biases corresponding to absolute humidity for each color toner etc., so that a processing condition or tone correcting value to be used is selected based on the absolute humidity measured by a temperature and humidity sensor (not shown).
In addition, in order to maintain a constant density, tone and color tint (which may be called image formation characteristics) irrespective of changes in conditions of portions of the apparatus during use, the apparatus forms toner patches for density detection on the intermediate transfer member with respective colors of toner, and detects the toner patches using an optical sensor disposed at a position similar to the color misregistration detecting means 22 so as to provide feedback to processing conditions such as an exposure amount or a developing bias to perform density control in order to ensure stable images.
However, use of conventional sensors for detection of the color tint of the toner patches on the sheet after the fixing or detection of the density of the toner patches on the intermediate transfer member for the purpose of obtaining stable images in the image forming apparatus has involved the following problems.
First, in the sensor utilizing a photodiode shown in FIG. 13A, since a photocurrent generated in a photo-receiving portion is directly subjected to I/V conversion (i.e. current-to-voltage conversion), it is difficult to secure a sufficient quantity of light that can create a sensor output having a good S/N ratio. When the toner density on the intermediate member is detected utilizing diffused reflected light from the toner patch, the regular reflected light is eliminated from the reflected light for detection, so the quantity of light available for the detection is reduced. In addition, in order to perform the xcex3-correction, it is necessary to detect reflected light from patches of various toner reflectances. Therefore, it is necessary to detect a patch(s) of low reflectance, in which case the quantity of incident light on the sensor is further reduced.
Furthermore, when the diffused reflected light is made to pass through color filters such as R (red), G (green) and B (blue) for selective detections in order to detect the color tint of the toner on the sheet, the quantity of light is still further reduced, since the wavelength range of the incident light on one pixel of the sensor is restricted. When detection is performed on light having been diffracted by a diffraction grating or the like, the quantity of light is greatly reduced, since the incident light on each pixel of the sensor has a more narrow wavelength range.
It is true that the voltage can be increased by increasing the value of the resistance used for the I/V conversion, but in that case, the S/N ratio cannot be significantly enhanced, since random noises generated in the resistance also increase or the sensor becomes susceptible to external noises. It is also true that the quantity of incident light on the sensor can be increased by converging the light with a lens, but in this case, the optical system becomes more complex and increases costs.
On the other hand, in the case of the accumulation type sensor shown in FIG. 14, a good S/N ratio can be obtained even if a sufficient quantity of light is not available, since it accumulates in the accumulation capacity the photocurrent generated in the light receiving portion and reads the accumulated charge. However, in this sensor, changes in incident light cannot be detected in real time, so it is not possible to detect the position of the leading edge of the toner patch to be detected. So the timing for starting the sensor accumulation must be determined based on the timing of forming the electrostatic latent image of the toner patch to be detected on the photosensitive drum. In this case, it is necessary to form a large size of patch, since there is a variation in the time required for the toner patch to be delivered to the detection area of the sensor. In the case in which the toner density is detected by detecting a toner patch on the intermediate transfer member, the enlargement of the toner patch size causes an increase in the time required for detecting the toner patch for toner density detection, so as to bring about problems such as a decrease in the usability of the apparatus and economical inefficiency due to an increase in waste toner. On the other hand, in the case in which the toner density is detected by detecting a toner patch on the sheet after the fixing, the enlargement of the toner patch size causes a decrease in the number of the toner patches that can be formed on a sheet of a predetermined size, so as to bring about problems such as a decrease in detectable information and a decrease in accuracy in color stabilization control of the image forming apparatus.
The present invention has been made under the above-mentioned situation. It is an object of the invention to provide an image forming apparatus having good color reproductivity as well as a toner patch detection method, in which reflected light from a toner patch can be detected with a good S/N ratio based on a small quantity of light, and the color tint and density of the toner patch can be detected under limited time and a limited length of an intermediate transfer member or transfer material, without wasting the toner.
An image forming apparatus according to the present invention comprises image forming means for forming an image, patch forming means for causing said image forming means to form a patch for image formation characteristics detection and a patch for position detection, first detecting means for detecting a patch for image formation characteristics detection formed by said image forming means in order to control an image forming condition, second detecting means for detecting a patch for position detection, and controlling means for controlling, in response to the detection by said second detecting means, a detecting operation of said first detecting means.
According to the present invention, there is also provided a method for detecting a patch for controlling image formation characteristics of an image forming apparatus, using a first sensor and a second sensor, comprising a step of causing the image forming apparatus to form a patch for image formation characteristics detection and a patch for position detection, a step of detecting the patch for position detection formed by said image forming apparatus, using the second sensor, a step of controlling, in response to detection by said second sensor, a detecting operation of said first sensor, and a step of detecting the patch for image formation characteristic detection formed by said image forming apparatus based on detection by said first sensor so as to obtain information on image formation characteristics of said image forming apparatus.
This and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.